A fuel cell is a device for converting a change in free energy caused by oxidation reaction of fuel to electric energy. As examples of the fuel cell, there have been developed a phosphate fuel cell, a polymer electrolyte fuel cell, a molten carbonate fuel cell and a solid oxide fuel cell.
The solid oxide fuel cell is capable of generating electric energy at a high temperature of 600° C. to 1,000° C. Therefore, the solid oxide fuel cell enjoys highest energy conversion efficiency among the fuel cells developed thus far. Thanks to the high energy conversion efficiency, the solid oxide fuel cell, if practically available, can become a substitute for the existing energy converters. If hydrogen is used as the fuel of the solid oxide fuel cell, it is possible to reduce emission of carbon dioxide (CO2). It is therefore expected that the solid oxide fuel cell will be used as an energy source for a future energy system.
In the meantime, the solid oxide fuel cell is operable at a high temperature and is capable of causing reaction within a fuel electrode (anode). This provides an advantage in that the solid oxide fuel cell can use not only hydrogen but also other kinds of fuel such as a natural gas and a coal gas. Unlike the molten carbonate fuel cell, the solid oxide fuel cell does not use liquid electrolyte and does not suffer from problems of material corrosion, electrolyte loss and electrolyte supplement. Complex power generation can be performed through the use of high-quality waste heat dissipated from the solid oxide fuel cell. This makes it possible to enhance the efficiency of a power generation system as a whole.
The solid oxide fuel cell is composed of a unit cell including an oxygen ionic conductive electrolyte, an air electrode (cathode) and a fuel electrode (anode), latter two of which are arranged on the opposite surface of the electrolyte. If an air and reducing fuel such as hydrogen are supplied to the respective electrodes of the unit cell, reduction reaction of oxygen occurs in the air electrode, thereby generating oxygen ions. The oxygen ions move toward the fuel electrode through the electrolyte and react again with reducing fuel such as hydrogen supplied to the fuel electrode, thus generating water. At this time, electrons are generated in the fuel electrode and are consumed in the air electrode. Electricity can be obtained by interconnecting the fuel electrode and the air electrode.
The solid oxide fuel cell is largely divided into a tubular type and a planar type. The tubular type solid oxide fuel cell is disclosed in many different patent documents, e.g., KR10-0286779B and KR10-0344936B.
JP2004-31158A discloses a fuel cell in which a plurality of parallel channels is formed in an electrolyte support body and in which an air electrode and a fuel electrode are formed on the inner walls of each of the channels.
In the fuel cell disclosed in JP2004-31158A, a cap is arranged between an upper unit cell and a lower unit cell. The cap is provided to discharge an exhaust gas going through reaction in the unit cells. A plurality of discharge holes and an exhaust gas flow path are arranged in the cap so that an air and a fuel gas should not be mixed with each other. The fuel introduced passes through the fuel gas flow path, the discharge holes and the exhaust gas flow path. Then, the fuel is discharged to the outside. The air introduced passes through the air flow path, the discharge holes and the exhaust gas flow path. Thereafter, the air is discharged to the outside.
The cap has a plate shape. A rectangular exhaust gas flow path is formed in the central region of the cap. A plurality of discharge holes is formed in the position of the exhaust gas flow path corresponding to the fuel gas flow path or the air flow path. Thus, the cap has a structure substantially hard to manufacture.
The cap is not a power generating part. For that reason, there is a problem in that the power generation area per unit volume is reduced if the cap is arranged between the unit cells adjoining each other.
In the fuel cell disclosed in JP2004-31158A, the flow paths of upper and lower unit cells adjoining each other are not connected to each other. For that reason, the fuel introduced takes part in reaction only when passing through the channels of the unit cells of the respective layers. Thereafter, the fuel is discharged to the outside. In order to increase the reaction area of a fuel gas and an air and to enhance the power generation efficiency, it is necessary to increase the length of the fuel gas flow paths and the air flow paths of the unit cells, eventually increasing the size of the unit cells. In this case, however, the moving route of electrons becomes longer and the power generation efficiency grows lower. This poses a problem in that it is difficult to increase the power generation capacity. Moreover, a difficulty is involved in forming long channels in an electrolyte support body. It is also highly likely that defects are generated in the process of forming the long channels. In addition, there is a need to increase the size of the cap which is hard to manufacture.
Korean Patent No. 0815207 discloses a fuel cell in which a plurality of parallel channels is formed in an electrolyte support body and in which an air electrode and a fuel electrode are formed on the inner walls of the channels. In this fuel cell, passages are formed in the channels so as to interconnect air flow paths and fuel flow paths formed in upper unit cells and lower unit cells adjoining each other. With the structure of this fuel cell, electricity collecting plates have to be installed on the opposite side surface of each of the unit cells to which side plates are coupled. Therefore, each of the unit cells requires two electricity collecting plates. In general, the electricity collecting plates are made of an expensive metallic material. It is therefore necessary to reduce the number of the electricity collecting plates installed. Since the electricity collecting plates are installed on the opposite side surface of each of the unit cells, the distance from the air electrode and the fuel electrode formed on the inner surfaces of the channels distant from the side surfaces to the electricity collecting plates becomes longer. This poses a problem of increasing the moving distance of electrons.